Not Applicable
Not Applicable
Not Applicable
The present invention relates to vehicle power assist steering systems and particularly to systems of the type providing a variable assist to the vehicle operator in response to changing conditions, as for example road speed. Such systems for providing variable steering effort assist in a vehicle have employed a solenoid operated flow control valve to control flow of hydraulic fluid to a power steering actuator, typically a hydraulic piston and cylinder type actuator. Presently, it is known to provide a solenoid operated flow control valve having a single inlet and two outlets with one outlet connected to the steering actuator and the other outlet connected to a pressure control valve or bleed valve for maintaining a controlled supply pressure to the power steering actuator wherein the flow through the pressure control valve is inversely proportioned to the solenoid energization current.
This arrangement has been necessitated in variable assist power steering systems because of the range of pressures experienced from the supply pump which has its speed varying with the engine shaft speed. In such a variable assist power steering system arrangement, the pressure control valve serves to bleed hydraulic fluid from the flow control valve to the inlet of the pump or sump when the pressure in the flow control valve valving chamber exceeds a pre-selected value thereby enabling the solenoid operated valve to control flow to the steering actuator at a generally controlled pressure.
Heretofore, solenoid operated valves for variable assist power steering assist systems have provided a flow response which is decreased with increasing current flow to the solenoid from a vehicle on-board electronic controller which received an input from a vehicle speed sensor. The electronic controller provided an electrical current to the solenoid for controlling flow to the steering actuator. However, at high current levels and low flow the position of the valve pintle close to the valve seat created a Bernoulli effect which permitted the pressure acting on the pintle head to drive the valve suddenly to a closed position maximizing the power assist to the steering and creating a sudden change in the steering response. In such an arrangement, where a failure occurs in the power supply to the solenoid during operation, a sudden decrease in steering effort required by the vehicle operator could result in the operator""s over steering the vehicle and create a hazardous condition
It is known to provide a solenoid pressure balanced spool valve for constant level assist power steering systems as described in U.S. Pat. 5,607,137. However, spool valves have a limited range of flow response and may not be suitable for variable assist power steering applications.
Accordingly, it has been desired to provide a solenoid operated proportional flow control valve for a hydraulic power steering system which gives the desired flow control, particularly at low current levels to the solenoid and which eliminates the effect of the pressure forces which tend to drive the valve to a suddenly closed position.
It therefore has been desired to provide a solenoid operated flow control valve for a variable assist power steering system with flow to the steering actuator directly proportional to the solenoid current, yet prevent sudden valve closing and to provide a residual or minimum flow with the solenoid de-energized.
It is an object of the present invention to provide a flow control valve which provides flow from an inlet to a first outlet which is proportional to the energization current of the solenoid actuator of the valve and bleed flow to control pressure is provided at a second outlet.
It is another object of the invention to provide a solenoid operated flow control valve having the flow directly proportional to the solenoid energization current and to have the valve configured as a pintle-type valve to provide a relatively low manufacturing cost to render the valve suitable for high volume mass production vehicle variable assist power steering systems.
It is a further object of the present invention to provide a solenoid operated proportional flow control valve which is able to maintain a low or residual flow in the de-energized state to the first outlet without the pressure forces closing the valve.
The solenoid operated flow control valve of the present invention provides a poppet passing through the valve seat to the inlet side and having a reverse taper to provide directly proportional flow and a through passage in the poppet which communicates the inlet pressure with the opposite end of the poppet to balance the pressure forces acting on the poppet in the axial direction, thus eliminating the tendency of the pressure forces to abruptly close the valve at low flow positions of the poppet. The present invention provides calibration of the valve in the de-energized state to provide a residual low flow to the control signal port. The valve of the present invention is particularly suitable for variable assist power steering applications in motor vehicles.